Essential advantages of a pure vertical evaporator tube concept are simple construction of the burning chamber suspension means, low manufacturing and assembly outlay and relatively great ease of maintenance. In comparison with a burning chamber wall with spiral tubes, the investment costs can be reduced considerably in this way. Owing to the design, however, the temperature imbalances of evaporator tube concepts of this type with perpendicular tubes are substantially greater in comparison with burning chambers with spiral tubes. Whereas the evaporator tubes in a spiral winding run through virtually all the heating zones of the burning chamber and a satisfactory heating equalization can therefore be achieved, the individual burning chamber tubes of the perpendicular tubes remain in the respective heating zone from the upstream evaporator inlet header to the downstream evaporator outlet header. Therefore, tubes in greatly heated burning chamber regions, for example in the vicinity of the burners or else in the middle wall region of burning chambers with a rectangular cross section, experience continuous additional heating over the entire tube length. Tubes in weakly heated burning chamber regions, in particular the corner wall tubes of the burning chamber with a rectangular cross section, experience less heating over the entire tube length in contrast. In designs with spiral evaporator tubes, the additional and lesser heating of individual tubes or tube groups lies in the low single-digit percent range. In the case of designs with perpendicular tubes, in contrast, considerably greater additional and lesser heating in relation to the mean heat absorption of an individual evaporator tube is known. Accordingly, the essential challenge in the case of burning chamber walls with perpendicular tubes lies in the ability to control said great heating imbalances between individual evaporator tubes.
A way of solving the above-described problem which is very effective and has already been disclosed in DE 4 431 185 A1 is a design of the perpendicular tubes according to what is known as the “low mass flux” design. In this approach, lowest possible mass flow densities which result in a positive throughput characteristic of the individual evaporator tubes are aimed for in the perpendicular tubes. Specifically, this means that tubes with more heating have a higher throughput and tubes with less heating have a lower throughput. Therefore, the occurrence of impermissibly high temperature imbalances can be counteracted effectively solely by way of a targeted application of the laws of physics. Since, however, the requirements with regard to the degree of efficiency of the facilities have risen constantly in the last years and therefore the live steam temperature and pressure have likewise increased continuously and, in addition, ever greater load ranges also have to be covered by way of the power plant facility, there is a necessity to further develop said “low mass flux” design.
The use of newly developed materials and the ability to manage them during processing and during operation of the power plant facility additionally make it necessary to reduce possible temperature imbalances still further.
It would be obvious to divide the mass flow distribution to individual burning chamber wall regions and therefore different groups of evaporator tubes and then to manipulate this in a targeted manner. Specifically, this means that wall regions with high heating are in particular to have comparatively great throughflow rates and wall regions with low heating are to have correspondingly lower throughflow rates. For this purpose, the burning chamber has to be divided into representative wall regions in order to take different heating zones into consideration. This takes place by way of a segmentation of the inlet and outlet headers. Here, each header segment is assigned to a wall region with the representative heating. In the inlet region, each header segment is provided with a dedicated feed water supply line. By way of the selection of a suitable geometric configuration of said supply lines, or by way of the installation of additional orifice plates in the region of said supply lines, the division of the entire feed water mass flow to the individual header segments can be performed in a targeted manner depending on the respective heating situation.
Supply lines or orifice plates which are adapted to one another geometrically have the decisive disadvantage, however, that their throttling action changes with the load. Therefore, the mass flow distribution in the evaporator and the associated temperature imbalances at the evaporator outlet can be optimized only for a defined load range owing to the system. Moreover, both the supply lines and the orifice plates can be designed in a targeted manner and adapted to one another only in the case of precise knowledge of the heat distribution over the burning chamber circumference. If the heat distribution which occurs then differs during operation of the power plant facility from the distribution which is used in the design calculations of the supply lines or orifice plates, the temperature imbalances can even rise in the most unfavorable case. The idea of further securing the design via the geometric adaptation of the supply lines with or without orifice plates is therefore even reversed in some circumstances.